The present invention relates to a lithium ion secondary battery, and more specifically, to an improvement in the level of the battery safety.
With recent improvement in performance of electronic appliances, a battery used for a power source of electronic appliances, especially a rechargeable secondary battery has been demanded to improve performance thereof. The lithium ion secondary battery is paid attention to, because it can drive electronic appliances for longer hours, be light and portable, and have high capacity. In spite of the advantage of high energy density, the lithium ion secondary battery is required to provide a sufficient measure for safety because lithium metal and a non-aqueous electrolyte are used.
As a measure for safety it has been conventionally suggested to incorporate a safety valve which releases increased internal pressure, or a PTC element which increases resistance in accordance with the heat generated from external short circuit to break an electric current.
For example, as disclosed in Japanese Unexamined Patent Publication No.328278/1992, a safety valve and a PTC element are attached to the positive electrode cap of a cylindrical battery. However, the safety value is generally designed not to operate easily because its operation may cause water contained in the air to invade into a battery to react with lithium in the negative electrode.
On the other hand, the PTC element successively breaks an external short-circuit without causing any troubles. As a safety component running firstly at the emergency of the battery, the PTC element can be designed to run when the battery reaches a temperature of at least 120xc2x0 C. due to a short circuit.
By the way, at occurrence of a short-circuit inside the battery, breaking of the external circuit by the PTC element does not mean the breaking of the short-circuit inside the battery. When the short-circuit inside the battery increases a temperature of the battery, a polyethylene or polypropylene separator disposed between the positive electrode and the negative electrode is expected to melt and release or confine a non-aqueous electrolyte contained therein to decrease its ion conductivity, thereby reducing the short-circuit current.
However, the separator away from the heating part does not always melt. To solve the problem, Japanese Unexamined Patent Publication No.161389/1995 proposed use of positive electrode active material particles having a PTC property.
However, resistance of the positive electrode active material having the PTC property is about 10xe2x88x925 S/cm at a temperature in use (around a room temperature), the battery does not function unless the positive electrode is formed by adding a conductive aid to the positive electrode active material having the PTC property as disclosed in Example. Addition of a conductive aid having no PTC properties allows the short-circuit current to flow via the conductive aid when the positive electrode active material has a PTC property.
The present invention has an object to solve the above-mentioned problems and to provide a lithium ion secondary battery with a high level of safety to reduce heat generated by external and internal short-circuits.
The first lithium ion secondary battery of the present invention comprises a plurality of electrode laminates formed by arranging a separator holding an electrolyte, a first electrode and a second electrode, wherein the electrodes have an active material on both sides of the separator, an electronic conductive material in contact with the active material, and an electronic conductive current collector jointed with the active material and the electronic conductive material with a binder, and wherein the active material, the electronic conductive material, or the electronic conductive material current collector of at least one of the electrodes has property of increasing resistance with increasing a temperature. Consequently, at occurrence of a short-circuits inside the battery, the short-circuit current can be reduced automatically by PTC function of either the active material and the electronic conductive material in the route of the short-circuit current or the electronic conductive material used for the positive electrode current collector to reduce an increase of a temperature.
The second lithium ion secondary battery of the present invention is that in the first lithium ion secondary battery a plurality of the electrode laminates are formed by alternately arranging the first electrode and the second electrode between a plurality of divided separators and jointing them with an adhesive agent.
The third lithium ion secondary battery of the present invention is that in the first lithium ion secondary battery a plurality of the electrode laminates are formed by alternately arranging the first electrode and the second electrodes between a coiled separator and jointing them with an adhesive agent.
The fourth lithium ion secondary battery of the present invention is that in the first lithium ion secondary battery a plurality of electrode laminates are formed by alternately arranging the first electrode and the second electrode between a folded separator and jointing them with an adhesive agent.
Since the electrode laminates are formed by adhesive in the present invention, no strong outer can is needed when the structure has a plurality of electrode laminates, a laminated electrode-type compact battery can be obtained, which is compact and safe and has large battery capacity.
The fifth lithium ion secondary battery of the present invention is that in the first lithium secondary battery the active material comprises a part having an electrode activity and a part having no electrode activity, and the part having no electrode activity increases resistance with increasing a temperature. Thereby, the temperature rise can be reduced by increasing reactive resistance of the active material at an occurrence of a short-circuit.
The sixth lithium ion secondary battery of the present invention is that in the first lithium secondary battery the active material is secondary particles comprising a plurality of active material particles having on their surfaces electronic conductive particles of which resistance increases with increasing a temperature. Thereby, a temperature rise can be reduced by increasing reactive resistance of the active material at an occurrence of a short-circuit.
The seventh lithium ion secondary battery of the present invention is that in the first lithium secondary battery the electronic conductive current collector has a property of increasing resistance with increasing a temperature, and the electrode active material layer containing the active material comprises a plurality of parts electronically insulated and separated. Thereby, it is prevented that at an occurrence of a short-circuit, the active material layer becomes a bypass of the short-circuit current to disturb a decrease in the short-circuit current.
The eighth lithium ion secondary battery of the present invention is that in the first lithium secondary battery the electronic conductive current collector comprises a conductive plate and an electronic conductive material which is jointed with the conductive plate and has a property of increasing resistance with increasing a temperature.
The ninth lithium ion secondary battery of the present invention is that in the eighth lithium secondary battery the conductive plate comprises a metal.
The tenth lithium ion secondary battery of the present invention is that in the eighth lithium secondary battery the conductive plate comprises carbon.
Thereby, the battery can be provided with a PITC property, which restricts an increase in a temperature due to an occurrence of a short-circuit inside the battery and offer a high level of safety.
The eleventh lithium ion secondary battery of the present invention is that in the first lithium secondary battery the electronic conductive material is a polymer having a softening temperature of at most 150xc2x0 C. Thereby, the heat generated by the short-circuit melts a plastic, and the active material has larger electronic conductive resistance by interrupting both the electronic conductive route and the ion conductive route. When it is used for an electronic conductive material, the electronic conductive route can be interrupted.